1. Field of the Invention
This invention relates to metallurgy and to zirconium and hafnium metallic compositions, and in particular to a method for making very high purity reactive metal sponge.
2. Description of the Prior Art
In the commercial production of zirconium and hafnium metal, the ore is generally initially subjected to a chlorination step which produces a relatively impure, hafnium containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easy separated). The hafnium and zirconium containing material is then subjected to a number of purifying operations and also a complex hafnium separation operation. These operations result in purified oxides of zirconium and hafnium, which, of course, are maintained separate. The purified oxides are then separately chlorinated. Zirconium and hafnium are commonly reduced from the chloride by means of a reducing metal such as magnesium. At the present time, the commercial processes are batch-type processes. U.S. Pat. No. 3,966,460, for example, describes a process of introducing zirconium tetrachloride vapor onto molten magnesium, with the zirconium being reduced and traveling down through the magnesium layer to the bottom of the reactor and with the by-product magnesium chloride being periodically removed. In the commercial processes, however, the by-product salt (e.g. magnesium chloride) remains till the batch is completed and cooled. The salt and metallic sponge (zirconium or hafnium) are then removed from the reduction vessel. A portion of the salt is manually removed. The metallic sponge (containing remaining salt and some remaining excess reducing metal) is then placed in a distillation vessel for removal of the remaining salt and magnesium by high temperature vacuum distillation.
Combination reduction and distillation furnaces have been proposed in the past, as well as arrangements for intermediate tapping of magnesium chloride, for example, in U.S. Pat. No. 2,787,539 to Kunklin, issued Apr. 2, 1957. Intermediate tapping of magnesium chloride together with a separate vessel for feeding zirconium tetrachloride is taught in U.S. Pat. No. 3,715,205 to Ishizuka on Feb. 6, 1973.
Molten salt systems for purification but not for directly feeding reduction of zirconium tetrachloride have also been proposed, in U.S. Pat. No. 2,916,362 to Horrigan and 3,057,682 to Groce, with Groce in addition proposing adding finely divided zirconium metal for greater purification. Zirconium and hafnium have also been purified by iodide cells to produce so-called "crystal bar" material. This is a rather expensive step which is performed after reduction and is discussed, for example, in U.S. Pat. No. 4,368,072 issued to Siddall on Jan. 11, 1983.
Ultrapure zirconium has been proposed for a liner for the inside surface of Zircaloy tubing which is used as cladding for nuclear fuel and is described in, for example, U.S. Pat. No. 4,372,817 to Armijo et al on Feb. 8, 1983. A similar use of a moderate purity material is proposed in U.S. Pat. No. 4,200,492 to Armijo on Apr. 29, 1980.